A way to improve opamp clipping?

[Bill Mountain]

I'm working on a rat-ish pedal and one of the modes will be a no diodes mode.  I like the extra headroom but unless the gain is suuuuppppperrrr high the clipping opamp sounds quite bad (as expected).

I have been toying with the idea of boosting the supply voltage (high enough that it no longer becomes a factor) and then adding clipping diodes to the feedback path of the opamp to total somewhere around 6VP-P to sort of mimic the headroom of the opamp but with slightly better sounding clipping.

Is it really that simple or is there more to it?

I've experimented with a few LEDs in series but I couldn't tell if they were working or not.  They wouldn't light up and the sound was the same (to me atleast) when I removed them.

[midwayfair]

Are you using the right chip? I mean, the op amp clipping is the main, uhhhh, "charm" of the RAT. Only certain op amps sound good in that thing.

Even if you make the voltage the stated maximums of the chip (CHECK THE DATASHEET), you will have gobs more gain than the headroom will allow. I'm not sure it's possible to avoid op amp clipping in the RAT, never mind that it turns it into a different pedal. You're probably better off just switching to a design that was made to do what you're doing ... the BJFe Honey Bee is almost exactly what you've described: LED clipping in the feedback loop, hard clipping after.

[samhay]

+1 to what Jon said. A variation on the theme would be to add diodes to ground after the op-amp stage that have a high enough forward voltage (LEDs perhaps) such that they won't conduct if your feedback diodes are clipping, but will conduct if you are clipping the op-amp rails.

[ashcat_lt]

Are you using the right chip? I mean, the op amp clipping is the main, uhhhh, "charm" of the RAT. Only certain op amps sound good in that thing.

In a normal Rat, the clipping diodes conduct way before the opamp does, and any opamp clipping is pretty well hidden, or kinda just bonus.  The big impact that the opamp has is in its frequency response - GBW and slew rate.  That will make a difference with or without the diodes, but I don't really think the clipping itself is particularly important.

Even if you make the voltage the stated maximums of the chip (CHECK THE DATASHEET), you will have gobs more gain than the headroom will allow. I'm not sure it's possible to avoid op amp clipping in the RAT, never mind that it turns it into a different pedal. You're probably better off just switching to a design that was made to do what you're doing ... the BJFe Honey Bee is almost exactly what you've described: LED clipping in the feedback loop, hard clipping after.

The feedback diodes actually reduce the gain of that stage toward unity.  The signal will still swing beyond the "limits" set by those diodes, but not likely more than a volt or two with a typical guitar input.  An 18V rail would give plenty of headroom with 6V worth of diodes.  I'm not completely convinced that it'll sound a whole lot better this way, but there's no real problem with the theory.

My Rat works fine with the clipping diodes removed, but then I'm always running it into something that's going to distort on its own well before the opamp in the pedal hits the rails.  Why are you asking it for such a huge output if not to push something downstream into overdrive?

[GGBB]

Not sure how similar to a stock RAT your circuit is, but if you've got the stock JFET before the output, you could be clipping that if you lift the diode clippers, which may be what is not sounding good. The 2N5458 gate is biased to ground and its cutoff voltage may be anywhere from -1 to -7. In the Turbo RAT, the JFET is biased at 1/2 Vin to allow for the greater headroom of the LED clippers. A simple mod is to tack a 1M resistor between drain and gate so as to bias it to 1/2 Vin. Early models of the Turbo RAT actually did this on the back of the PCB (but using 2.2M) before they redesigned the layout.

[ashcat_lt]

Not sure how similar to a stock RAT your circuit is, but if you've got the stock JFET before the output, you could be clipping that if you lift the diode clippers, which may be what is not sounding good. The 2N5458 gate is biased to ground and its cutoff voltage may be anywhere from -1 to -7. In the Turbo RAT, the JFET is biased at 1/2 Vin to allow for the greater headroom of the LED clippers. A simple mod is to tack a 1M resistor between drain and gate so as to bias it to 1/2 Vin. Early models of the Turbo RAT actually did this on the back of the PCB (but using 2.2M) before they redesigned the layout.

This occurred to me right after I hit the post button.